1. Field of the Invention
This invention relates to a liquid level detecting device for detecting the liquid level of a sample solution in the course of sucking up the sample solution in a vessel by use of a suction member. This invention particularly relates to a liquid level detecting device for use in an apparatus for automatically applying a predetermined amount of a sample solution onto a chemical analysis slide provided with a predetermined reagent layer.
2. Description of the Prior Art
Qualitative or quantitative analysis of a specific chemical constituent in a liquid sample is widely conducted for various industrial purposes. Particularly, it is very important in the biochemical and clinical fields to quantitatively analyze chemical constituents or physical constituents in body fluid such as blood or urine.
In recent years, as disclosed in, for example, Japanese Patent Publication No. 53(1978)-21677 and Japanese Unexamined Patent Publication No. 55 (1980)-164356, there has been developed and put into practice a dry type chemical analysis slide for quantitatively analyzing a specific chemical constituent or a specific physical constituent contained in a sample solution simply by applying to the slide a droplet of the sample solution. With the chemical analysis slide, it is possible to analyze a sample solution more simply and more quickly than when the conventional wet type analysis method is used. Therefore, the use of the chemical analysis slide is desirable particularly in medical institutions, research laboratories, or the like where many samples have to be analyzed.
When a chemical constituent or the like contained in a sample solution is to be analyzed by use of the chemical analysis slide, a measured amount of the sample solution is put on the chemical analysis slide and is maintained at a predetermined temperature (i.e. incubated) for a predetermined time in an incubator to cause a color reaction, and the reflection optical density is measured by exposing the chemical analysis slide to measuring light having a wavelength selected in advance in accordance with the combination of the constituent of the sample solution and the reagent contained in the reagent layer of the chemical analysis slide. In this manner, it is possible to achieve quantitative analysis of the chemical constituent or the like.
In the case where the aforesaid analysis is to be carried out, a predetermined amount of the sample solution must be measured accurately and applied to the reagent layer of the chemical analysis slide. This is because, if the amount of the sample solution applied differs from the predetermined amount, the optical density of the reflected light becomes different from the correct value, and the accuracy of the analysis is degraded. Accordingly, various pipettes have heretofore been proposed for accurately applying a predetermined amount of the sample solution when the sample solution is to be applied. For example, one of the proposed pipettes is constituted so that an application tip is fitted to a lower end of the pipette, a predetermined amount of a sample solution is sucked into the application tip, and then the predetermined amount of the sample solution is applied from the application tip onto a reagent layer of a chemical analysis slide. In general, with such a pipette, the predetermined amount of the sample solution is sucked into the application tip and discharged therefrom by use of a piston and cylinder mechanism. When the sample solution is to be sucked into the application tip and discharged onto the reagent layer by use of the pipette, the lower end of the application tip is first inserted into the sample solution, the predetermined amount of the sample solution is sucked in and kept in the application tip by use of a piston and cylinder mechanism or the like, then the lower end of the application tip is positioned above the reagent layer of the chemical analysis slide, and the sample solution in the application tip is applied onto the reagent layer by use of the piston and cylinder mechanism. However, in this case, when the lower end of the application tip is inserted into the sample solution and then pulled out therefrom for sample application, the sample solution is also taken up on the outer circumferential portion of the lower end of the application tip and applied onto the reagent layer together with the sample solution sucked in the application tip. As a result, error arises with the amount of the sample solution applied to the reagent layer, and the accuracy of measurement is degraded.
In order to eliminate the aforesaid problems, after the sample solution is sucked into the application tip by the pipette, the sample solution taken up on the outer circumferential portion of the lower end of the application tip has heretofore been wiped off, and then the sample solution sucked in the application tip is applied onto the reagent layer. However, in this case, a wiping operation is necessary and the operation efficiency becomes low. Also, when sample application is to be carried out automatically, it is not always possible to achieve the wiping operation automatically.
In order to solve the aforesaid problems, the applicant proposed an automatic sample applying apparatus constituted so that, when a sample solution is sucked into an application tip, the vertical position of the application tip and the liquid level of the sample solution are controlled to always make constant the insertion distance of the lower end of the application tip into the sample solution and to always make constant the amount of the sample solution taken up on the outer circumferential portion of the lower end of the application tip, thereby to always make constant the amount of the sample solution applied to the reagent layer of a chemical analysis slide. In this case, in order to make constant the insertion distance of the lower end of the application tip into the sample solution, it is necessary to accurately detect the position of the lower end of the application tip and the liquid level of the sample solution. However, in general, the application tip is frequently exchanged with a new application tip for preventing the sample solution from being mixed with a different sample solution, for hygienic reasons, or the like. Also, in general, the dimensional accuracy of the application tip is not so high, and application tips having different shapes may be used. Accordingly, it is not always possible to accurately detect the position of the lower end of the application tip fitted to the lower end of the pipette, and to control the insertion distance of the lower end of the application tip into the sample solution. Also, regarding the liquid level of the sample solution, it is not easy to accurately detect the liquid level in the case where, for example, bubbles are present on the surface of the sample solution.